The present invention relates generally to abrasive articles. More particularly, the present invention relates to an abrasive article wherein the abrasive particles are affixed to a substrate with a corrosion resistant matrix material including a sintered corrosion resistant powder and a brazing alloy chemically bonded with the abrasive particles, thereby securely holding the particles in place, and further relates to a method of making such an abrasive article.
Abrasive articles, such as polishing or conditioning disks, are generally formed by affixing abrasive particles to a carrier or substrate with a matrix material. Such abrasive articles are used to smooth or polish the surface of a workpiece, such as a urethane pad, which may, in turn, be used to polish components, such as silicon wafers. Conditioning disks are used in a wide variety of environments including highly corrosive environments which degrade the structural integrity of the article. Thus, if the abrasive particles are not adequately secured to the substrate, the particles will have a tendency to become dislodged from the matrix material. Once dislodged, an abrasive particle can easily scratch and damage the polished surface of the workpiece. In addition, once one particle is dislodged, support for adjacent particles is decreased, and additional particles are more likely to become dislodged. Accordingly, a conditioning disk which maintains its strength, wear resistance, and structural integrity in a corrosive environment is highly desirable.
Various techniques have been used to affix abrasive particles to a substrate. Each technique includes surrounding the abrasive particles with a matrix material which forms a bond between the particles and substrate, thereby serving to hold the particles in place. One such known technique is electroplating which includes depositing a metal, typically nickel, to a thickness in the range of 40-75% of the height of the particle, thereby forming a bond with the abrasive particles which is a purely mechanical attachment. The Bruxvoort et al. U.S. Pat. No. 5,251,802, for example, discloses an abrasive article including a plurality of abrasive composites bonded to a backing. Each of the abrasive composites includes a plurality of abrasive grains, such as diamond or cubic boron nitride, and a preferably metallic binder of tin, bronze, nickel, silver, iron, and alloys or combinations thereof for securing the abrasive grains to the backing. The binder is applied to the backing by an electroplating process and the abrasive grains are applied simultaneously during the electroplating process. Electroplating is limited in that not all abrasive particles form adequate bonds with electro-deposited metal. In addition, not all metals are capable of electrodeposition, therefore limiting the range of metallic compositions which can be used in the electroplating process.
Another known technique for affixing abrasive particles to a substrate is by sintering the matrix material. Sintering involves applying heat and/or pressure to a fusible matrix material containing abrasive particles, thereby serving to affix the abrasive particles to the substrate. The Tselesin U.S. Pat. No. 5,380,390, for example, discloses an abrasive article and method in which the abrasive particles are affixed to a substrate by a sinterable or fusible matrix material. The Lowder et al. U.S. Pat. No. 5,511,718 discloses a process of brazing diamond to create monolayer tools with a nickel-chromium-boron alloy. While sintering generally serves to affix the abrasive particles to the substrate, the abrasive particles have a tendency to become dislodged from the matrix material during operation, particularly in a corrosive environment. Thus, there exists the need for a corrosion resistant abrasive article in which the abrasive particles remain affixed to the substrate over extended periods of operation under adverse operating conditions.
The present invention provides an abrasive article for use in a corrosive environment, and a method of making such an abrasive article. More particularly, the present invention provides an abrasive article in which the abrasive particles are affixed to one or both sides of a substrate using a corrosion resistant matrix material which forms a chemical bond as well as a mechanical attachment with the abrasive particles, thereby securely holding the particles in place on the substrate in a wide variety of operating conditions. The substrate may be a separate component to which the abrasive particle and matrix material composite is affixed, or the substrate may be formed integrally of matrix material.
The size and type of abrasive particles are selected to achieve the desired characteristics of the abrasive article depending on its intended application. The term xe2x80x9cabrasive particlesxe2x80x9d includes single abrasive particles bonded together by a binder to form an abrasive agglomerate or composite. Abrasive agglomerates are further described in U.S. Pat. No. 4,311,499 to Kressner, U.S. Pat. No. 4,652,275 to Bloecher et al., and U.S. Pat. No. 4,799,939 to Bloecher et al. The abrasive particles may further include a surface treatment or coating, such as a coupling agent or a metal or ceramic coating. Abrasive particles useful in the present invention have an average size of generally 20 to 1000 micrometers. More specifically, the abrasive particles have an average size of about 45 to 625 micrometers, or about 75 to 300 micrometers. Occasionally, abrasive particle sizes are reported in terms of xe2x80x9cmeshxe2x80x9d or xe2x80x9cgrade,xe2x80x9d both of which are commonly known abrasive particle sizing methods. It is preferred that the abrasive particles have a Mohs hardness of at least 8 and, more preferably, at least 9. Suitable abrasive particles include, for example, fused aluminum oxide, ceramic aluminum oxide, heat treated aluminum oxide, silicon carbide, boron carbide, tungsten carbide, alumina zirconia, iron oxide, diamond (natural and synthetic), ceria, cubic boron nitride, garnet, carborundum, boron suboxide, and combinations thereof.
In accordance with a characterizing feature of the invention, the matrix material includes a brazing alloy and a sintered corrosion resistant powder. When heated to a pre-determined temperature, the brazing alloy becomes liquid and flows around the abrasive particles. In addition, the brazing alloy reacts with and forms a chemical bond with the abrasive particles. In order to form the chemical bond, the composition of the brazing alloy includes a pre-selected element known to react with the particular abrasive particle, thereby forming the chemical bond. For example, if diamond abrasive particles are used, the brazing alloy may include at least one of the following elements which may react and form a chemical bond with the diamond: chromium, tungsten, cobalt, titanium, zinc, iron, manganese, or silicon. By way of further example, if cubic boron nitride abrasive particles are used, the brazing alloy may include at least one of aluminum, boron, carbon and silicon which may form the chemical bond with the abrasive particles, and if aluminum oxide abrasive particles are used, the brazing alloy may include at least one of aluminum, boron, carbon, and silicon. It will be recognized, however, that the brazing alloy may also contain various inert elements in addition to the element or elements which react with and form the chemical bond with the abrasive particles.
A quantity of corrosion resistant powder is admixed with the brazing alloy to improve the bonding properties, enhance the strength, improve the corrosion resistant properties, and reduce the cost of the matrix material. The corrosion resistant powder may include metals and metal alloys including stainless steel, titanium, titanium alloys, zirconium, zirconium alloys, nickel, and nickel alloys. More specifically, the nickel alloy can include nichrome, a nickel alloy including 80% nickel and 20% chrome by weight. Alternatively, the corrosion resistant powder can be formed of ceramics including carbides, such as silicon or tungsten carbide.
In one embodiment, the substrate is formed of stainless steel and is affixed to a support carrier in the form of a stainless steel shim using an epoxy film. It will be apparent, however, that both the substrate and carrier may be formed of other materials such as, for example, synthetic plastic materials, ceramic materials, or other suitable corrosion resistant metals. It will also be apparent that the substrate and carrier can be connected with any suitable fastening technique including adhesive or mechanical fasteners.
In another embodiment of the invention, the carrier is formed of a polycarbonate material, such as LEXAN(trademark), and has a generally annular shape with a plurality of gear teeth included along its outer edge surface. The abrasive particles and matrix material are formed into abrasive segments which are affixed directly to the carrier with suitable fastening means. Each segment includes an abrasive portion containing the abrasive particles and an in situ substrate portion formed entirely of matrix material.
To reduce the likelihood of abrasive particles breaking loose from the substrate in the region where the substrate is cut to the desired shape, the portion of the substrate which is cut may be provided free of abrasive particles. This particle free zone may, for example, extend a certain distance along the entire edge of the substrate. For a typical conditioning disk having a generally circular or annular shape, the particle free zone is provided at the outer peripheral edge portion of the substrate. Depending on the application, abrasive particles can be provided on one or both sides of the substrate.
The present invention further provides a method of fabricating an abrasive article in which the abrasive particles are affixed to a substrate with a corrosion resistant matrix material including a brazing alloy and a corrosion resistant powder. The method includes first applying a layer of matrix material to the substrate and then arranging the abrasive particles in the matrix material so that a portion of each abrasive particle is surrounded by matrix material. The abrasive particles are arranged on the substrate to provide a particle free zone, thereby eliminating the problem of having abrasive particles in that zone becoming loose as a result of weakness caused by the cutting process. Next, the matrix material is treated with heat and/or pressure to cause the brazing alloy to become liquid and flow between the abrasive particles and between the interstices of the corrosion resistant powder. During this step the brazing alloy forms a chemical bond with the abrasive particles, and forms an inter-metallic compound at the interface with the corrosion resistant powder, thereby bonding the brazing alloy with the corrosion resistant powder. In addition, the combination of heat and pressure causes the corrosion resistant powder to sinter.
During the heating and pressurizing step, the article is heated to a temperature in the range of generally between 500 and 1200 degrees Celsius and pressurized to a pressure in the range of generally between 75 and 400 kg/cm2, and is maintained at this temperature and pressure for a time period sufficient to allow the brazing alloy to form the chemical bond with the abrasive particles, to allow the brazing alloy to form the inter-metallic compound with the corrosion resistant powder, and to allow the powder to sinter. A time period of generally between 3 and 15 minutes has been found to be sufficient.
A more specific method of applying heat and pressure to the article includes covering the abrasive particles and matrix material with a layer of material such as, for example, graphite paper, which is electrically conductive and conforms to the contours of the abrasive surface. This method requires the additional step of removing the conductive layer using known techniques such as, for example, sandblasting, pressure washing with water, high pressure waterjet cleaning, or chemically dissolving the layer to expose the abrasive particles following the heat and pressure treatment.
The method of forming the invention may also include the additional steps of cutting the article through the particle free zone to a desired shape such as, for example, an annular disk shape; flattening the article; cleaning the article; and attaching the article to a carrier.